tailwindcss/src/lib/formatCSS.js

function indentRecursive(node, indent = 0) {
  node.each &&
    node.each((child, i) => {
      if (!child.raws.before || child.raws.before.includes('\n')) {
        child.raws.before = `\n${node.type !== 'rule' && i > 0 ? '\n' : ''}${'  '.repeat(indent)}`
      }
      child.raws.after = `\n${'  '.repeat(indent)}`
      indentRecursive(child, indent + 1)
    })
}

export default function formatNodes(root) {
  indentRecursive(root)
  root.first.raws.before = ''
}